How do I invite users into my firm's account?

You'll need to be a Super User.

Go to https://designerpages.com/firm/users and click the "Invite User" button in the upper right corner. A light-box will appear prompting you to fill in the user's email address and the corresponding office. Then click the "Invite User" button. Next, the user will receive an email to complete their registration and access your firm's account.